The present invention relates to a process for the production of diazonium compounds with a low content of sodium ions and the use of the resulting compounds for the preparation of azo compounds, and in particular azo dyes.
Most dye applications require dyes which are highly soluble in water. Some salt forms of dyes containing sulfo or carboxy groups are for example less water-soluble than others. The corresponding sodium salts or sodium mixed salts, such as for example Na/K salts, frequently have particularly low solubility. Where possible it is therefore desirable to prepare dyes from other salt forms, such as for example lithium, potassium or ammonium salts. Particularly in the case of dye salts of relatively low solubility conversion into the corresponding more highly water-soluble salt forms is very complicated since high dilution rates are required and considerable quantities of effluent result.
Attempts are therefore made to use starting materials which are already in free from or have only a low content of sodium ions in the coupling reaction for the preparation of corresponding azo compounds, and in particular azo dyes.
Diazonium compounds, which are generally obtained by the diazotization of the corresponding amines with sodium nitrite in the presence of inorganic acids, are frequently obtained in the form of water-insoluble compounds which can be filtered off and then washed with water until free of sodium ions. This process is problematical from the point of view of safety, since the filter cake can dry onto the membrane and, on exposure to impact or heat, the explosive decomposition of the diazonium compounds, which are known to be unstable in the dry state, is then likely to occur.
A process has now been found which avoids the above-mentioned disadvantages.